The blue bezel turns on the gauge so that you can set the pressure range as it relates to the outside temperature. Here is how it works: determine the outside temperature around your car. Turn the bezel so that the arrow points to the outside temperature. Notice the “V” shape on the dial.
This is the appropriate pressure range based on the ambient (outside) temperature. When reading the pressure, if the arrow is below the range then charge the car until the needle on the pressure gauge is in the range. If it is above the upper end of the range your system is overcharged. In this case you should seek the help of a qualified technician to recover refrigerant from your system to bring the system back into the proper range. Never vent A/C refrigerant into the atmosphere. It is harmful to the environment and illegal to do so.